In a transfer machine, a workpiece is supported at a position by a jig or a pallet, and held immoved at every station by them for necessary cutting operation by a processing unit.
In most cases, a processing unit of this type includes a numerical control device to automatically index the position of a tool with respect to a workpiece to effect cutting operation there under a necessary feeding amount (processing depth).
However, thermal deformation often occurs in feed screws, spindle, column or base of the processing unit during working operation. Beside this, there often exist dimensional errors in the jig itself of the transfer machine, positional errors of a workpiece with respect to the jig and cumulative errors among multiple parts or members, and they necessarily invite a positional error between the workpiece and the processing unit.
Therefore, even if the processing unit precisely controls the positions and the feeding amount according to a predetermined numerical control program, the aforegoing thermal deformation and dimensional errors in various parts inevitably invite a working error of the workpiece.
In this connection, the present inventors formerly proposed a technology disclosed in Laying-Open Publication No. 61-16254 of Japanese Utility Model application in which after the positional relationship between a workpiece and a tool of a spindle is indexed by a sensor of the spindle located at a position distant from the workpiece by a predetermined distance, the working depth is numerically controlled.
This proposal certainly ensures a high precision working. However, upon fixing the position of the spindle in its feeding direction, i.e. during a motion of the sensor for detecting a reference position, the spindle is temporarily held in non-feeding condition for the purpose of rotational position indexing. This is a loss time which decreases the processing capacity of the equipment.